This invention relates to the measurement of small magnetic fields produced by the body, and, more particularly, to stimulating tactile sensations in the body so that the resulting magnetic fields produced by the body may be measured.
The biomagnetometer is a device that measures the very small magnetic fields produced by the body. The magnetic fields, particularly those produced by electrical currents flowing in the brain and the heart, can be important indicators of the health of the body, because aberrations in the magnetic field can be associated with certain types of disfunctions either for diagnosis or early prediction. Moreover, the magnetic fields produced by the brain are an indicator of thought processes and the location at which such processes occur, and can be used to investigate the mechanisms of thought.
Magnetic fields produced by the body are very small, because they result from very small electrical current flows. Typically, the strength of the magnetic field produced by the brain is about 0.00000001 Gauss. by comparison, the strength of the earth's magnetic field is about 0.5 Gauss, or over ten million times larger than the magnetic field of the brain.
The biomagnetometer must therefore include a very sensitive detector of magnetic fields. Current biomagnetometers utilize a pickup coil which produces an electrical current when a magnetic field penetrates the pickup coil. The electrical current, which is typically very small in magnitude, is detected by a Superconducting QUantum Interference Device, also known by the acronym SQUID. Spurious effects from the detection of other magnetic fields than those produced by the brain can be removed by appropriate filters. However, the ability of filters to remove all of the extraneous effects is limited. To further improve the signal-to-noise ratio of the system, the subject and pickup coil can be located in a magnetically shielded room. The operation of SQUIDS and their electronics are disclosed in U.S. Pat. Nos. 3,980,076; 4,079,730; 4,386,361; and 4,403,189. A biomagnetometer is disclosed in U.S. Pat. No. 4,793,355. A magnetically shielded room is disclosed in U.S. Pat. No. 3,557,777. The disclosures of all of these patents are incorporated herein by reference.
The biomagnetometer is often used to measure the magnetic fields produced by the brain either spontaneously (without an external stimulus) or in response to an external stimulus. In the latter case, the external stimulus can be a visual stimulus, an auditory stimulus, a tactile (touch) stimulus, or other stimulus of interest. The present invention is concerned with the production of tactile stimuli and the measurement of the response of the body to such stimuli.
It is common practice to induce tactile stimulation by attaching a small electrode to the body of the subject, and controllably applying a small voltage to the electrode. The body of the subject reacts to the small electrical shock, and that response is measured by the biomagnetometer. Although this approach is operable in one respect, it produces a particular shock response rather than a mechanical touch response which may be of interest. Also, the established technique requires that an electrical signal be conducted into the magnetically shielded room. While the current is small, it is sufficient that the resulting magnetic field can be detected by the biomagnetometer. The detection of this stimulating signal interferes with the measurement of the biomagnetic signal resulting from the stimulation. A shock, even if relatively small, can be uncomfortable for the subject, particularly if the shock is applied in a sensitive region such as the face. Apprehension of the shock may itself produce measurable biomagnetic signals.
There is a need for an improved approach for controllably inducing tactile stimulation in the body of a person, in such a manner that there is no magnetic field produced that interferes with the biomagnetic measurement of the response to the stimulus. The present invention fulfills this need, and further provides related advantages.